It is known that subsurface organic contaminants and associated degradation products can occur as gas phase constituents in soil gas. In the case of releases of light nonaqueous (LNAPL) phase liquids, the mass flux of carbon dioxide (CO2), a common degradation product, provides an indicator of losses of LNAPL through natural attenuation processes. Natural attenuation of LNAPL bodies can occur at rates that rival or exceed conventional LNAPL recovery technologies. In fact, NAPL losses to the gas phase by volatilization and biodegradation may be as much as two orders of magnitude larger than those due to dissolution into groundwater. Biodegradation, largely driven by methanogenesis, may overcome the dominant LNAPL mass loss process over time as the more biodegradable volatile components are quickly lost from the NAPL. Numerical modeling and field measurements using multilevel gas samplers show that degradation-generated methane can be converted to carbon dioxide relatively quickly in the subsurface, and that greater than 98% of the carbon produced by biodegradation exits the ground surface as CO2.
Estimates of LNAPL losses may be used in mass balance calculations to indicate whether LNAPL bodies are stable, expanding, or shrinking, and with the relative efficiencies of the various remedial alternatives, appropriate technologies may be selected for a site.
In the case of releases of potentially stable organic contaminants, such as chlorinated solvents, the flux of stable parent compounds, for example from soil gas into indoor air (vapor intrusion), may be of importance since the risks associated with exposure to impacted indoor air can depend on the flux or contaminant loading.
Active soil gas sampling (using vacuum collection of gas samples) is presently used for rapid screening of concentrations of volatile organic compounds (VOCs) in a subsurface with moderately permeable soils. Passive sampling, which relies on diffusion and absorption, can also be used to sample for both VOCs and semi-volatile organic compounds. Tubes containing absorbent material may be placed in a sampling matrix near the surface where equilibrium has been allowed to develop between the soil gases and the sorbent over several days to weeks, and analyzed for the absorbed gas of interest, generally in a laboratory. Granular adsorbent materials in a chemically inert, hydrophobic, microporous expanded polytetrafluoroethane membrane, wherein vapors may move freely across the membrane and onto the sorbent material while water and soil are prevented from entering the sampler, have been used to identify chlorinated and aromatic vapors migrating to the surface. While passive samplers provide an indication of concentrations, they do not provide information regarding the mass of contaminants crossing a given cross-sectional area over time (contaminant flux)
Carbon dioxide efflux at the ground surface has been measured using dynamic closed chamber (DCC) method. Contaminant-related soil respiration (CSR) may be calculated from natural soil respiration (NSR) measured at background locations and total soil respiration (TSR) rates measured above LNAPL-contaminated regions. A correlation has been found between increased CO2-effluxes in regions containing crude oil as determined by core analyses.